Frame relay protocol-based multiplex switching scheme for satellite mesh network

ABSTRACT

A frame relay protocol-based earth station interface architecture provides full mesh connectivity for a relatively small number of network stations. The fundamental component of the architecture is a frame relay protocol-based switch, which employs a network interface ‘frame relay’ standard to define the multiplexing of multiple virtual ports across single physical communications port. Through address and control fields of its connectivity control software, the frame relay protocol-based switch can be dynamically configured to provide multilayer addressing and device selectivity, thereby enabling point-to-point connectivity of multiple terminal devices, such as a plurality of audio circuits, to be effected via a single port. Dial codes on the station side of an audio signal multiplexer link are translated into frame relay addresses (data link connection identifiers) that are added to each frame of data for routing through the network. With this additional layer of routing information, audio (voice) connectivity is now available between any two audio circuits (e.g. trunks) in the network.

FIELD OF THE INVENTION

[0001] The present invention relates in general to satellitecommunication systems, and is particularly directed to a frame relayprotocol-based earth station interface for providing full mesh,bidirectional signalling capability between a plurality of (diversebandwidth) end terminal devices, including multiple audio (voice)circuits, for any of the stations of the network.

BACKGROUND OF THE INVENTION

[0002] The increasing availability of reasonably priced satellitecommunication services, and a variety of narrow bandwidth (voice/data)and wide bandwidth (video) devices to meet the needs of a broad spectrumof communication system users, has led to communication systemarchitectures that can be tailored in terms of connectivity structureand customer utilization. This diversity of equipment types and signalprocessing capability has led to the desire to have ‘local’ areanetworks (LANs), customarily limited to a terrestrial-based systems,typically limited to geographical area, be expanded to encompass a muchlarger scale of communication services, preferably those employingsatellite link transmission equipment to connect terminal devices amongwell dispersed office sites.

[0003] To facilitate inter-office communications, it is preferred tohave such satellite-based systems configured as full mesh networks,diagrammatically illustrated in FIG. 1, where any terminal device 10 inthe network (comprised of a non-limitative example of four earthstations 11, 12, 13, 14 in the illustrated example) has a directsatellite link 20 (via one hop through a relay satellite 30) to anyother terminal device 10 in the network. Connectivity between arespective terminal device 10 that is ported to an associated stationinterface and a respective terminal device ported to another stationinterface may be effected by providing each earth station with amultiplexing, demultiplexing subsystem, that is operative tocontrollably uplink messages from any terminal device (e.g. audio(voice), data, video equipment) over an outbound link and to distributedownlink messages to their intended destination terminal devices.

[0004] One type of multiplexing scheme that might be used could involvea time division multiplexing (TDM) and demultiplexing arrangementthrough which a fixed number of bytes for each user port would beallocated within a fixed information frame. The frame size (total numberof bytes) may be determined by the number of ports and their data rates,and the number of frames transmitted per second. The number of TDMframes per second determines the aggregate data rate. The aggregate datarate includes the total user port data rate plus framing overhead.

[0005] Interfacing respective terminal devices with the TDM subsystemmay be effected by means of a dedicated multiport switch associated withthe respective multiplexer and demultiplexer units of the earth station,with each multiport switch being configured for an equal number of datacommunications equipment (DCE) and data terminal equipment (DTE) ports,so as to provide full matrix capability between DCE and DTE ports. Theport speed and format (DCE to DTE) must match; however, matrix switchescan usually translate between different physical and electricalcharacteristics.

[0006] A problem associated with such a TDM-matrix switch earth stationarchitecture proposal is the fact that its terminal-to-terminalconnectivity involves dedicated port connections, which remain fixedunless the system is physically reconfigured. As a result, in such assystem, only a very limited selectivity for voice calls is afforded,since only point-to-point connections can be effected between voicemultiplexers and not among the voice circuits themselves that connect tothe voice multiplexers. In addition, TDM schemes are very sensitive totiming and network synchronization, since no queuing is performed. Amaster network timing source is required for all network subsystems.Also, because suppliers of multiplexer and matrix switch components arenot the same, different monitor and control mechanisms are required foreach respective piece of equipment. This requirement is further burdenedby the fact that, due to the unique character of a simplex data stream,the required multiplexer/demultiplexer is not an off-the-shelf product.Finally, the cost of such a system is not insubstantial, since each ofthe multiport switch and the multiplexer and demultiplexer componentsmust be purchased separately.

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention, the desire to providefull mesh connectivity for a relatively small number of network stations(e.g. on the order of sixteen or less, as a non-limitative example) issuccessfully addressed by a frame relay protocol-based earth stationinterface architecture. The fundamental component of this architectureis a frame relay protocol-based switch, or simply frame relay switch,which comprises a multiplex communication component recently introducedfor use in voice/facsimile communication multiplex applications, andwhich employs a network interface ‘frame relay’ standard to define themultiplexing of multiple virtual ports across single physicalcommunications port. The interface standard ‘frame relay’ is based uponthe transmission and reception of individual frames or packets ofinformation serially through a port, with a respective frame of digitaldata containing additional address and control bytes for routing andelementary error detection and flow control.

[0008] In the novel earth station environment of the present invention,the frame relay switch is ported, via a first set of terminal ports, toa plurality of ‘local’ terminal devices, which may include respectivevoice, data and video signalling equipments. A voice signal linktransports low bit rate digitized voice signals, such as those having anencoding rate of less than 10 kb/s, to and from a voice signalmultiplexer, in order to interface voice traffic with a plurality ofvoice signalling circuits that are selectively accessible through themultiplexer. The voice signalling link also conveys call supervisionsignals, including dial tone detection, dialing, circuit busy, callconnect and call termination control and status signals. The voicesignal multiplexer is operative to append and decode terminal deviceselectivity information to the address field portion of a frameprocessed by the frame relay switch.

[0009] Also ported to the frame relay switch are one or more data linksthat may be coupled to two-way synchronous data terminal devices,providing data rate signalling on the order of 256 Kb/s, for example. Anadditional port of the frame relay switch may be coupled to a link forwide bandwidth signals, such as a video teleconferencing terminal. Theteleconferencing video and its associated voice signals may be digitizedand compressed into a single data stream at aggregate data rates on theorder of from 112 to 384 kb/s, for example. Because of the widerbandwidth required for video teleconferencing capability, the videocommunication port of the frame relay switch is intended to be used ononly an occasional basis, and may require one or more other signallingchannels to be turned off during the teleconferencing period.

[0010] Through address and control fields employed by frame relayconnectivity control software, the frame relay switch can be dynamicallyconfigured to provide multilayer addressing and device selectivity(filtering), thereby enabling point-to-point connectivity of multipleterminal devices, such as a plurality of voice circuits served by thevoice circuit multiplexer unit to which a voice signal port of the framerelay switch is coupled. Dial codes on the trunk or station side of thevoice signal link are translated into frame relay addresses (data linkconnection identifiers) that are added to each frame of data for routingthrough the network. With this additional layer of routing information,voice connectivity is now available between any two voice terminaldevices (e.g. trunks) in the network.

[0011] On its satellite link side, the frame relay switch is ported to aplurality of modulator and demodulator circuits contained within amodulator/demodulator unit. To provide full mesh connectivity among themultiple earth station network, the circuits of themodulator/demodulator unit include a single uplink modulator and aplurality of downlink demodulators. The respective modulator anddemodulator components may comprise PSK signalling units employing, forexample, (data rate dependent) BPSK/QPSK/MSK modulation. The modem unitis coupled to an attendant RF transceiver unit that is coupled to anassociated satellite antenna unit for transmitting uplink channelsignals to the relay satellite and receiving downlink channel signalsfrom the satellite.

[0012] In order to optimize traffic flow among the diversity of terminaldevices (voice, data, video) served by the frame relay-based interfaceof the present invention, the routing control mechanism employed by theframe switch relay's microcontroller includes priority queuing, whichprovides a plurality of queuing levels to control queuing delay throughthe frame relay switch. Voice frames are given highest priority, videoteleconferencing frames are given the next highest priority, and dataframes are given lowest priority. The queuing mechanism is defined suchthat during normal operation, the frame relay switch will not have moreoffered traffic than the aggregate outbound channel can handle. Priorityqueuing has effectively no impact on the sequence of transmitted frames.Where the offered load increases or the channel error rate exceedsprescribed limits, the priority queuing mechanism is operative to reducethe load impact on video teleconferencing first and then voicesignalling traffic.

[0013] Since, in a full connectivity mesh network, each earth station iscontinuously monitoring each downlink channel for message frames thatmay be addressed to it, it is desirable to provide a mechanism forreducing signal processing housekeeping that would otherwise be executedon data frames that are not intended for a destination terminal deviceserved by that earth station. The port configuration parameters of theframe relay switch define a bit mask, which is employed by themicrocontroller to ‘filter’ and selectively discard or pass frames basedupon a portion of or the entirety of the first byte of the frame relayaddress. This mask feature allows only downlinked frames from multipleinbound channels that are destined for one or more terminal devicesserved by that earth station to be accepted and processed by the framerelay switch. This preliminary filtering reduces processing load andincreases efficiency of the routing through the frame relay switch.

[0014] The address and routing mechanism employed by the frame relayswitch's microcontroller also inserts, within the frame relay header, adiscard eligibility bit, which signifies to the frame relay networkwhether or not, during periods of congestion, that frame can beinitially discarded in an attempt to alleviate the congestion condition.As a result of potential system congestion related to the abovedescribed priority queuing and filtering mechanisms, a prespecified datalink connection identifier may be employed to ‘force’ the discardeligibility bit in the frame relay header to a ‘one’ bit for all framesutilizing that particular data link connection identifier. This forcingof the discard eligibility bit to a ‘one’ by means of a data linkconnection identifier provides an extra level of control on framesoriginating from terminal devices that may be unable to set the discardeligibility bit themselves.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 diagrammatically illustrates a full mesh satellite-basedcommunication network, where any terminal device in the network has adirect satellite link to any other terminal device in the network;

[0016]FIG. 2 diagrammatically illustrates the architecture of a framerelay protocol-based earth station interface in accordance with anembodiment of the present invention;

[0017]FIG. 3 illustrates the field format of a frame relay frame; and

[0018]FIGS. 4, 5 and 6 show details of respective address, data linkconnection identifier (DLCI) and frame check sequence (FCS) fields ofthe frame relay format of FIG. 3.

DETAILED DESCRIPTION

[0019] Before describing in detail the particular frame relayprotocol-based earth station interface in accordance with the presentinvention, it should be observed that the present invention residesprimarily in a novel structural combination of conventional(commercially available) signal processing circuits and components andnot in the particular detailed configurations thereof. Accordingly, thestructure, control and arrangement of these circuits and components havebeen illustrated in the drawings by readily understandable blockdiagrams which show only those specific details that are pertinent tothe present invention, so as not to obscure the disclosure withstructural details which will be readily apparent to those skilled inthe art having the benefit of the description herein. Thus, the blockdiagram illustrations of the Figures do not necessarily represent themechanical structural arrangement of the exemplary system, but areprimarily intended to illustrate the major structural components of thesystem in a convenient functional grouping, whereby the presentinvention may be more readily understood.

[0020] Referring now to FIG. 2, the architecture of a frame relayprotocol-based earth station interface in accordance with an embodimentof the present invention is diagrammatically illustrated as comprising aframe relay protocol-based switch (or simply frame relay switch) 110,having a plurality of bidirectional signal coupling ports and serving tocontrollably interface signal transport links, that are coupled to aplurality of terminal devices, with modulator and demodulator componentsassociated with a satellite communication RF transceiver unit.

[0021] More particularly, frame relay switch 110 has a first pluralityof physical device, or terminal, ports 112, that are coupled to aplurality of ‘local’ terminal devices via respective terminal devicelinks, such as a voice signal multiplexer link 130, a plurality of datalinks 140, 150 and a video link 160. Voice signal link 130 transportslow bit rate digitized voice signals, such as those having an encodingrate of less than 10 kb/s, with echo cancellation and minimal queuingdelay to and from a voice signal multiplexer 135. Voice signalmultiplexer 135, in turn, is coupled to a plurality of voice signallingcircuits. The ports of multiplexer 135 which provide connectivitybetween one of plural voice signalling circuits and voice signal link130 are, in effect, virtual ports of the frame relay switch 110, sincelink 130 is physically connected to a single port of multiplexer 135 andnot to the terminal devices themselves.

[0022] For incoming signals from a respective voice circuit, multiplexer135 is operative to selectively couple signals sourced from that voicecircuit terminal device (e.g. trunk circuit) to voice signal link 130.In the course of multiplexing such a selected voice circuit to voicesignal link 130, multiplexer 135 provides data link connectionidentifier information (the virtual port address) as part of the addressfield of the call message signals being supplied to the frame relayswitch. The destination address field also contains a terminal device(e.g. called party number) code that a voice circuit multiplexer servedby a destination station employs to control the demultiplexing of thevoice signals to the called terminal device.

[0023] Similarly, in the course of demultiplexing an incoming callsupplied from frame relay switch 110 via voice signal link 130,multiplexer 135 decodes the data link connection identifier informationas part of the address field of the call message signals being suppliedfrom the frame relay switch, so as to controllably direct the call tothe intended terminal device. Also carried via link 130 are conventionalcall supervision signals, including dial tone detection, dialing,circuit busy, call connect and call termination control and statussignals.

[0024] Data links 140 and 150 may be coupled to two-way synchronous dataterminal devices, and may provide data rate signalling on the order of256 Kb/s. Video link 160 may be coupled to a video teleconferencingterminal. The teleconferencing video and its associated voice signalsmay be digitized and compressed into a single data stream at aggregatedata rates on the order of from 112 to 384 kb/s, for example. Because ofthe wider bandwidth required for video teleconferencing capability, thevideo communication port of the frame relay switch is intended to beused on only an occasional basis, and may require one or more othersignalling channels to be turned off during the teleconferencing period.

[0025] Frame relay switch 110 may comprise a commercially availableframe relay switch unit, such as a model 9800, microcontroller-driven,frame relay switch, manufactured by Teleglobe Inc., Montreal, Canada.The frame relay switch employs the network interface ‘frame relay’standard (e.g. ANSI, pending CCITT), to define the multiplexing ofmultiple virtual ports across single physical communications port. Theinterface standard ‘frame relay’ is based upon the transmission andreception of individual frames (or packets) of information seriallythrough a port. In accordance with this standard, a respective frame ofdigital data contains address and control bytes that are employed forrouting and elementary error detection and flow control.

[0026]FIG. 3 illustrates the field format of a frame relay frame, ascomprising an n octet frame including a first frame boundary flag (octet1=01111110), a sixteen bit address field comprised of address octets 2and 3, a user data field comprised of octets 3- n-3, a sixteen bit framecheck sequence (FCS) occupying octets n-2 and n-1, and a terminal frameboundary flag (octet n=01111110).

[0027] The respective components of the address field of the frame relayframe format of FIG. 2, of which octets 2 and 3 are comprised arediagrammatically illustrated in FIGS. 4 as comprising a first data linkconnection identifier (DLCI) comprised of bits 3-8 of octet 2, a(currently unused) bit 2, an extended address bit 1, a second data linkconnection identifier (DLCI) comprised of bits 5-8 of octet 3, a forward(towards the destination device) explicit congestion notification bit 4,a backward (from the sourcing device) explicit congestion notificationbit 3, a discard eligibility bit 2 (to be described) and an extendedaddress bit 1. The bit mapping for each data link connection identifier(DLCI) is shown in FIG. 5, while FIG. 6 shows the bit mapping for theframe check sequence.

[0028] As noted above, through the address and control fields of itsconnectivity control software, frame relay switch 110 can be dynamicallyconfigured to provide multilayer addressing and device selectivity(filtering), thereby enabling point-to-point connectivity of multipleterminal devices, such as a plurality of voice circuits 170-1, . . .170-N served by voice circuit multiplexer unit 135, to which voicesignal multiplexer link 130 is coupled, to be effected via a singleport. For this purpose, dial codes on the analog trunk or station sideof voice signal multiplexer link 130, which codes effectively representvirtual ports of the frame relay switch, are translated into frame relayaddresses (or data link connection identifiers) that are added to eachframe of data for routing through the network. With this additionallayer of routing information, voice connectivity is now availablebetween any two terminal devices (e.g. trunk circuits) in the network.

[0029] On its satellite link side, frame relay switch 110 is ported, viaa second set 114 of terminal ports, to a plurality of modulator anddemodulator circuits contained within a modulator/demodulator unit 120.To provide full mesh connectivity among the (four earth station) networkof the non-limitative example of FIG. 1, described above,modulator/demodulator (MODEM) unit 120 includes a single uplinkmodulator 210, and a plurality (three for the present example of a fourearth station network) of downlink demodulators 220, 230 and 240. Therespective modulator and demodulator components within MODEM unit 120may comprise PSK signalling units employing, for example, (data ratedependent) BPSK/QPSK/MSK modulation. Thus, frame relay switch 110provides for dynamic routing of signals between one or more terminaldevices to which ports 112 are coupled, and one or more modulators anddemodulators of MODEM unit 120 to which ports 114 are coupled.

[0030] MODEM unit 120 is coupled to an attendant RF transceiver unit 122that is coupled with a satellite uplink/downlink antenna unit 124. As anon-limitative example, the respective components of modem unit 120 mayinterface signals with RF transceiver unit 122 at a frequency on theorder of 70 MHz, while the satellite communication signals emitted byand received by RF transceiver unit 122 may fall within bandwidth of11-14.5 GHz. RF transceiver unit 122 may operate with time divisionmultiple access (TDMA) or single channel per carrier (SCPC) signallingformats.

[0031] In order to optimize traffic flow among terminal devices (voice,data, video equipments) served by the frame relay-based interface of thepresent invention, the routing control mechanism employed by the frameswitch relay's microcontroller also includes priority queuing, whichprovides a plurality (e.g. three for the three types of terminal devicesignalling services of the present example (voice, data, video)) ofqueuing levels to control queuing delay through the frame relay switch110. In particular, voice frames (ported via link 140) are given highestpriority, video teleconferencing frames (ported via link 160) are giventhe next highest priority, and data frames (ported via links 150) aregiven lowest priority. The queuing mechanism is defined such that duringnormal operation, the frame relay switch 110 will not have more offeredtraffic than the aggregate outbound channel can handle. Priority queuinghas effectively no impact on the sequence of transmitted frames. Wherethe offered load increases or the channel error rate exceeds prescribedlimits, the priority queuing mechanism is operative to reduce the loadimpact on video teleconferencing first and then voice signallingtraffic.

[0032] Since, in a full connectivity mesh network, each earth station iscontinuously monitoring each downlink channel for message frames thatmay be addressed to it, it is desirable to provide a mechanism forreducing signal processing housekeeping that would otherwise be executedon data frames that are not intended for a destination terminal deviceserved by that earth station. For this purpose, the port configurationparameters of the frame relay switch may be employed to define a bitmask, which is employed by the microcontroller to ‘filter’ andselectively discard or pass frames based upon a portion of or theentirety of the first byte of the frame relay address. This mask featureallows only downlinked frames from multiple inbound channels that aredestined for one or more terminal devices served by that earth stationto be accepted and processed by the frame relay switch. This preliminaryfiltering reduces processing load and increases efficiency of therouting through the frame relay switch.

[0033] The address and routing mechanism employed by the frame relayswitch's microcontroller contains, within the frame relay header, theabove-referenced discard eligibility (DE) bit (within the second octetof the address field, shown in FIG. 4), which signifies to the framerelay network whether or not, during periods of congestion, that framecan be initially discarded in an attempt to alleviate the congestioncondition. Namely, any frame whose discard eligibility bit has been setto a ‘one’ will be discarded in an attempt to alleviate the congestioncondition. As a result of potential system congestion related to theabove described priority queuing and filtering mechanisms, aprespecified data link connection identifier may be employed to ‘force’the discard eligibility bit in the frame relay header to a ‘one’ bit forall frames utilizing that particular data link connection identifier.This forcing of the discard eligibility bit to a ‘one’ by means of adata link connection identifier provides an extra level of control onframes originating from terminal devices that may be unable to set thediscard eligibility bit themselves.

[0034] As will be appreciated from the foregoing description, the framerelay-based earth station interface architecture in accordance with thepresent invention provides a mechanism for successfully achieving fullmesh connectivity for a relatively small number of network stations.Advantageously, since the fundamental component of the architecture is aframe relay switch, which employs a network interface ‘frame relay’standard to define the multiplexing of multiple virtual ports acrosssingle physical communications port. As a consequence, through addressand control fields of its connectivity control software, the frame relayswitch can be dynamically configured to provide multilayer addressingand device selectivity, thereby enabling point-to-point connectivity ofmultiple terminal devices, such as a plurality of voice circuits, to beeffected via a single port. Dial codes on the station side of an audio(voice) signal multiplexer link are translated into frame relayaddresses (data link connection identifiers) that are added to eachframe of data for routing through the network. With this additionallayer of routing information, audio (voice) connectivity is nowavailable between any two audio (voice) circuits (e.g. trunks) in thenetwork.

[0035] While I have shown and described an embodiment in accordance withthe present invention, it is to be understood that the same is notlimited thereto but is susceptible to numerous changes and modificationsas known to a person skilled in the art, and I therefore do not wish tobe limited to the details shown and described herein but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

What is claimed:
 1. A satellite communication network for providing fullmesh connectivity through a communication satellite, among terminaldevices that are coupled to associated earth stations comprising, at arespective earth station: a transceiver unit which is operative totransmit, via an up-link communication channel to said communicationsatellite, messages which are broadcast by said communication satelliteover downlink communication channels to respective earth stations ofsaid network, and which is operative to receive, via a respectivedownlink communication channel from said communication satellite,messages which are broadcast by said communication satellite over saiddownlink communication channels to respective earth stations of saidnetwork; a modulator/demodulator unit, which is coupled with saidtransceiver unit and is operative to modulate uplink-directed messages,supplied by one or more terminal devices that are coupled to said earthstation, in accordance with a prescribed modulation format fortransmission by said transceiver unit over said uplink channel, and todemodulate downlink-directed messages received by said transceiver unitfrom said communication satellite and destined for one or more terminaldevices that are coupled to said earth station; and a frame relayprotocol-based switch having a plurality of physical terminal deviceports coupled to communication links for interfacing messages to andfrom terminal devices served by said earth station, and a plurality ofuplink/downlink ports, coupled to said modulator/demodulator unit, saidframe relay protocol-based switch being operative to controllablyconnect a selected one of multiple virtual ports associated with atleast one of said physical terminal device ports to a selected one ofsaid uplink/downlink communications ports, and thereby connectuplink-directed message signals from any terminal device, that iscapable of having a communication path coupled to one of said multiplevirtual ports, to an uplink port of said plurality of uplink/downlinkports, and to controllably connect downlink-directed message signals,that are coupled to any downlink port of said plurality ofuplink/downlink ports, to any selected one of said virtual ports.
 2. Asatellite communication network according to claim 1, wherein one ofsaid plurality of physical terminal device ports is coupled to aterminal device multiplexer which is operative to controllably interfacesignals between a plurality of virtual ports to which plural terminaldevices served by said earth station are coupled and said one of saidplurality of physical terminal device ports.
 3. A satellitecommunication network according to claim 1, wherein one of saidplurality of physical terminal device ports is coupled to an audiosignalling circuit multiplexer which is operative to controllablyinterface audio message signals between a plurality of virtual ports towhich audio circuit devices served by said earth station are coupled andsaid one of said plurality of physical terminal device ports.
 4. Asatellite communication network according to claim 1, wherein saidmodulator/demodulator unit contains a single uplink modulator and aplurality of downlink demodulators.
 5. A satellite communication networkaccording to claim 1, wherein said terminal devices include respectiveaudio, data and video signalling devices.
 6. A satellite communicationnetwork according to claim 1, wherein said frame relay protocol-basedswitch is operative to route message signals therethrough in accordancewith a plurality of queuing levels respectively associated withdifferent types of messages signals.
 7. A satellite communicationnetwork according to claim 6, wherein said terminal devices includerespective audio, data and video signalling devices and wherein saidframe relay protocol-based switch is operative to route message signalstherethrough such that audio frames are given highest priority, videoteleconferencing frames are given the next highest priority, and dataframes are given lowest priority.
 8. A satellite communication networkaccording to claim 6, wherein port configuration parameters of saidframe relay protocol-based switch define a frame routing control codewhich is operative to selectively discard or pass frames based upon aprescribed portion of a frame relay address.
 9. A satellitecommunication network according to claim 8, wherein said portconfiguration parameters of the frame relay protocol-based switch whichdefine said frame routing control code are based upon contents of thefirst byte of the frame relay address.
 10. A satellite communicationnetwork according to claim 6, wherein a frame associated with aprescribed frame address processed by said frame relay-based protocolswitch contains a frame relay header that includes a discard eligibilitycode which causes said respective frame to be initially discarded in anattempt to alleviate a congestion condition.
 11. For use with asatellite communication network having a communication satellite and aplurality of earth stations, each earth station being coupled to aplurality of terminal devices, a method of providing full meshconnectivity between any two terminal devices of said network comprisingthe steps of: (a) providing, at each of said earth stations, atransceiver unit which is operative to transmit, via an up-linkcommunication channel to said communication satellite, messages whichare broadcast by said communication satellite over downlinkcommunication channels to respective earth stations of said network, andwhich is operative to receive, via a respective downlink communicationchannel from said communication satellite, messages which are broadcastby said communication satellite over said downlink communicationchannels to respective earth stations of said network, amodulator/demodulator unit, which is coupled with said transceiver unitand is operative to modulate uplink-directed messages, supplied by oneor more terminal devices that are coupled to said earth station, inaccordance with a prescribed modulation format for transmission by saidtransceiver unit over said uplink channel, and to demodulatedownlink-directed messages received by said transceiver unit from saidcommunication satellite and destined for one or more terminal devicesthat are coupled to said earth station, and a frame relay protocol-basedswitch having a plurality of physical terminal device ports coupled tocommunication links for interfacing messages to and from terminaldevices served by said earth station, and a plurality of uplink/downlinkports, coupled to said modulator/demodulator unit; (b) at a first earthstation sourcing a message from an associated calling terminal device,causing the frame relay protocol-based switch of said first earthstation to connect a selected one of multiple virtual ports associatedwith at least one of said physical terminal device ports to a selectedone of said uplink/downlink communications ports, and thereby connectuplink-directed message signals from said calling terminal device to anuplink port of said plurality of uplink/downlink ports; and (c) at asecond earth station terminating a message to an associated destinationterminal device, causing the frame relay protocol-based switch of saidsecond earth station to controllably connect downlink-directed messagesignals, that are coupled to a downlink port of said plurality ofuplink/downlink ports, to a selected virtual port to which saiddestination terminal device is coupled.
 12. A method according to claim11, wherein one of said plurality of physical terminal device ports iscoupled to a terminal device multiplexer which is operative tocontrollably interface signals between a plurality of virtual ports towhich plural terminal devices served by said earth station are coupledand said one of said plurality of physical terminal device ports.
 13. Amethod according to claim 11, wherein one of said plurality of physicalterminal device ports is coupled to an audio signalling circuitmultiplexer which is operative to controllably interface audio messagesignals between a plurality of virtual ports to which audio circuitdevices served by said earth station are coupled and said one of saidplurality of physical terminal device ports.
 14. A method according toclaim 11, wherein said modulator/demodulator unit contains a singleuplink modulator and a plurality of downlink demodulators.
 15. A methodaccording to claim 11, wherein said terminal devices include respectiveaudio, data and video signalling devices.
 16. A method according toclaim 11, wherein step (b) includes causing said frame relayprotocol-based switch to route message signals therethrough inaccordance with a plurality of queuing levels respectively associatedwith different types of messages signals.
 17. A method according toclaim 16, wherein said terminal devices include respective audio, dataand video signalling devices and wherein step (b) comprises causing saidframe relay protocol-based switch to route message signals therethroughsuch that audio frames are given highest priority, videoteleconferencing frames are given the next highest priority, and dataframes are given lowest priority.
 18. A method according to claim 16,wherein port configuration parameters of said frame relay protocol-basedswitch define a frame routing control code, and wherein step (c)comprises causing said frame relay protocol-based switch to selectivelydiscard or pass frames therethrough in accordance with said framerouting control code.
 19. A method according to claim 18, wherein saidport configuration parameters of the frame relay protocol-based switchwhich define said frame routing control code are based upon contents ofthe first byte of the frame relay address.
 20. A method according toclaim 16, wherein a frame associated with a prescribed frame addressprocessed by said frame relay-based protocol switch contains a framerelay header that includes a discard eligibility code in response towhich said respective frame is discarded by said frame relayprotocol-based switch in an attempt to alleviate a congestion condition.